Rust forms when oxygen reacts with iron, but simply putting iron and oxygen together isn't sufficient. Although about 21% of air consists of oxygen,  rusting doesn't occur in dry air. It occurs in moist air and in water. Rust requires three chemicals in order to form: iron, oxygen, and water. iron + water + oxygen → hydrated iron (III) oxide

The iron reacts with water and oxygen to form hydrated iron (III) oxide, which we see as rust. Here is the word equation for the reaction: iron + water + oxygen → hydrated iron (III) oxide Iron and steel rust when they come into contact with water and oxygen – both are needed for rusting to occur.

The chemical reaction for rusting is 4Fe + 3O2 + x H2O = 2Fe2O3.xH2O. Iron is first oxidized to iron (II) ions by oxygen. In the second step, the iron (II) ions are oxidized and combined with water and oxygen gas to produce a hydrated form of iron (III) oxide known as rust.

Iron rusting is an oxidation reaction. During rusting, iron combines with oxygen in the air in the presence of water to generate Fe2O3.xH2O, a hydrated iron (III) oxide. This hydrated iron (Ill) oxide is referred to as rust. Rust is largely hydrated Iron (III) Oxide, Fe2O3.xH2O. The color of rust is reddish-brown.